Billable is defined as the lesser of either $ Delivered or $ Ordered.  So, if a campaign over-delivers (say, due to more people listening at the time of a spot play than the campaign goal needs to deliver for the last spot play) - make sure to reference the $ Ordered number.  Since typically stations cannot bill more than was ordered.  If the campaign under-delivers or delivers exactly as needed - reference the $ Delivered number for a billable number.